Method of time and frequency synchronization of multimode terminal

ABSTRACT

A method of time-synchronizing at least two radio access modules of a multi mode communication terminal capable to function according to at least two distinct radio access techniques in a cellular telecommunication network in which one of the radio access modules is active in the current cell and the other radio access module is in a passive state in the current cell comprises the steps of: (a) measuring for each of the cells adjacent to the current cell a time offset T?offset#191 between the start of a specific frame of the first radio technique and the start of a specific frame of the second radio access technique, and (b) using the time offset T?offset#191 measured in step (a) for synchronizing the passive radio access module with the active radio access modules.

TECHNICAL FIELD

The invention is in the field of time and frequency synchronization of amultimode terminal with at least two different base stations. Morespecifically, the present invention relates to a method and a device fortime-synchronizing at least two radio access modules of a multimodecommunication terminal capable of functioning according to at least twodistinct radio access techniques in a cellular telecommunication networkin which one of the radio access module is active in the current celland the other radio access module is in a passive state in the currentcell.

The invention applies to a multimode terminal such as, for example, aGSM/UMTS (Global System for Mobile Communications, and Universal MobileTelecommunications System, respectively) dual-mode terminal.

BACKGROUND ART

Currently GSM is the mobile telephone system of reference in Europe andin the world. In order to ensure its evolution towards the thirdgeneration, ISO (International Organization for Standardization) hasproposed UMTS. Technical specifications of UMTS are developed under apartnership called 3GPP (Third Generation Partnership Project). Onething desired in the process of standardizing UMTS was to ensure itsinteroperability with GSM. Thus, a specific classification describingfour types of dual-mode GSM/UMTS terminals, Type 1, Type 2, Type 3, andType 4 was established.

For terminals of Type 1, when a terminal is located in a GMS radionetwork or in a UTRA (Universal Terrestrial Radio Access) radio network,the “inactive” radio part of the terminal (UTRA or GSM) does not performany measurement of reception quality, with the implication thattransition from one mode to the other can only be made with interventionof the user.

For terminals of Type 2, when a mobile terminal is connected to a GSMradio network or to a UMTS radio network, the inactive radio part of theterminal (UTRA or GSM) may perform measurements in order to switchautomatically to the network that provides the better reception quality.

Terminals of Type 3 are different from those of Type 2 in that theyfurther provide the possibility of receiving information simultaneouslyin both modes. On the other hand, simultaneous transmission in bothmodes is not possible. As with terminals of Type 2, transition from oneradio network to another is performed automatically.

For terminals of Type 4, transition from one radio network to another isperformed automatically, and information may be received and transmittedsimultaneously over the two networks.

The present invention finds applications in terminals of Type 2 as wellas in those of Type 1. Note that among the four types of terminals ofthis classification, it is the dual-mode terminals of Type 2 that allowthe operators to make current investments in GSM profitable, takingadvantage of the existing coverage while familiarizing their subscriberswith new services provided by UMTS.

At the level of functional architecture, a Type 2 terminal comprises anelectronic card responsible for performing tasks specific to GSM and anelectronic card that performs tasks specific to UMTS. In this type ofterminal, at a given moment, one card is necessarily active and theother is inactive. Consequently, at that moment, only communication withthe network associated with the active card (GSM or UMTS) is possible.The inactive card can only perform measurements on neighboring cellsbelonging to the other radio access network (GSM or UMTS). Themeasurements possibly serve for switching automatically from a GSMnetwork to a UMTS network and vice-versa.

Recall that in a single-mode terminal (GSM or UMTS), in order to avoidrelative frequency error between the reference clock in the electroniccard (GSM or UMTS) and the clock of the base station of the network (GSMor UMTS), it is necessary for the terminal to be synchronized with theassociated network in time and frequency to be able to decode and thendemodulate information coming from the base station. This error may haveseveral causes among which are variations of temperature or even agingof components of the terminal.

In order to compensate for this error, a very common technique used inthe prior art called AFC (Automatic Frequency Control) consists inensuring frequency synchronization with the current base station to beas precise as possible with the aid of a closed loop that is slaved to areference frequency value. The loop is supplied with estimations offrequency errors measured on the downlink channels that the mobile islistening to.

The object of the invention is to update the time base of the passiveradio access module with respect to that of the active radio accessmodule. The time base defines a whole set of counters of time-measuringunits used in each radio access technology.

Another object of the invention is to optimize the autonomy of themobile terminal by maintaining one of the radio access modules in the“sleep” state as much as possible while preserving its synchronizationwith the network by means of the active radio access module. This shouldallow the inactive radio access module to perform measurements onneighboring cells of the same radio technique in time windows allocatedby the current radio network.

DISCLOSURE OF THE INVENTION

The present invention proposes a method of time synchronization of radioaccess modules of a multimode communication terminal capable tocommunicate with base stations of a cellular telecommunication networkvia a plurality of distinct radio access networks, each based on atleast a first radio access technique and a second radio access techniquedifferent from the first radio access technique, one of the radio accessmodules being active in a cell of the network called current cell, whilethe other radio access modules being passive in that cell.

The method according to the invention comprises the following steps:

(a) measuring for each of the cells adjacent to the current cell a timeoffset T_(offset) between the start of a specific frame of the firstradio technique and the start of a specific frame of the second radioaccess technique; and

(b) using the time offset T_(offset) measured in step (a) forsynchronizing the passive radio access module with the active radioaccess modules.

The method according to the invention may further comprise a stepconsisting in activating the passive radio access module from the activeradio access module when measurements of power should be performed bythe passive radio access module.

The method according to the invention may further comprise a stepconsisting in updating the value of the offset T_(offset) on each changeof the current cell.

According to a preferred embodiment of the invention, the mobileterminal is a UMTS/GSM dual-mode terminal and the predefined durationT_(offset) is the time difference observed on a GSM cell defined in thestandard 3GPP TS 25.215.

The invention thus makes it possible to take advantage of themeasurement specified in the UMTS standard called “Observed timedifference to a GSM cell” or “OTD.” This measurement is used by the UMTSnetwork for placing in time sequence compressed frames used forperforming measurements of power on GSM cells. It represents the timeoffset between the start of the UMTS superframe of the current cellhaving a size of 4096 frames of 10 ms and the start of a GSM multiframeof a neighboring GSM cell having 51 frames of 4.615 ms.

Typically, this measurement is performed upon request from the network.However, within the present invention, the “observed time difference toa GSM cell” will be measured as needed by the terminal.

According to the invention, activation of the passive radio accessmodule may be performed just before performing measurements onneighboring cells of the current cell.

The invention also relates to a device designed to implement the methodaccording to the invention, the device comprising:

means for measuring a time offset T_(offset) between the start of aspecific frame of the first radio technique and the start of a specificframe of the second radio access technique; and

means for generating a clock signal of the passive radio access moduleshifted with respect to the clock signal of the active radio accessmodule concerning the duration T_(offset.)

The invention also relates to a multimode mobile terminal comprising:

a radio access module dedicated to each operating mode;

a clock generator associated with each radio access module; and

a unit for calculating a time offset T_(offset) between the start of aspecific frame of a first operating mode and the start of a specificframe of a second operating mode in a cellular telecommunicationnetwork.

This terminal further comprises a central interface capable to generatea clock signal in the passive radio access module shifted with respectto the clock signal of the active radio access module by the durationT_(offset.)

According to the invention, the central interface may comprise a modulefor generating an order for activating the passive radio access module.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will be apparentfrom the following description, taken by way of non-limiting example,with reference to accompanying drawings in which:

FIG. 1 schematically represents a device designed to implement themethod according to the invention;

FIG. 2 schematically represents a particular embodiment of the unit forcalculating the difference of time observed in a GMS cell defined in thestandard 3GPP TS 25.215;

FIG. 3 represents a timing chart allowing to schematize the time offsetperceived by the mobile terminal between the starts of the UMTSsuperframe having a size of 4096 frames and the start of the GSMmultiframe having 51 frames; and

FIG. 4 schematically shows the way in which the passive radio accessmodule (GSM) is awaken by the active radio access module (UTRA) for thepurpose of examining neighboring GSM cells.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be described in an application in a UMTS/GSMdual-mode mobile terminal set to a UMTS cell by way of example. Thus,the active radio access module is UTRAN (Universal Terrestrial RadioAccess Network or UMTS Terrestrial Radio Access Network), and thepassive radio module is that of the GSM system.

In FIG. 1, a device for time-synchronizing UTRAN and GSM is shown. Thedevice comprises: a UTRA radio access module 2 connected to a clock 4 of19.2 MHz; a GSM radio access module 6 connected to a clock 8 of 13 MHz;and a time base module and generator 12 comprising a first unit 14designed to maintain the clock signal of the UTRA module 2, a secondunit 16 designed to maintain the clock signal of the GSM module 6, and aunit 18 for calculating the duration T_(offset) representing thedifference of time observed on GSM defined in the standard 3GPP TS25.215.

Each of elements described above is connected to a central interface 20programmed for generating a clock signal calculated from the 19.2 MHzUTRA clock signal, the 13 MHz GSM clock signal, and the time offsetT_(offset). The central interface 20 is further connected to the centralprocessing unit (CPU) 21 of the mobile terminal. The device is furtherprovided with a clock 10 of 32 KHz and the clock signal of 32 KHz issupplied to the UTRA module 2, GSM module 6 and time base generator 12.

With reference to FIG. 2, the calculation unit 18 comprises a counter 22connected to the first unit 14 and the second unit 16, and receives fromthe central interface 20 a command signal to start the counting at thestart of the UMTS superframe of the current UMTS cell having a size of4096 frames to the start of a GSM multiframe of a neighboring GSM cellhaving 51 frames.

In FIG. 3, the start of the UMTS superframe is illustrated by arrow 24,and the start of the GSM multiframe of a neighboring GSM cell having 51frames is illustrated by arrow 26.

Note that the measurement of the duration T_(offset) is performed onlyonce at the moment the mobile terminal is set to the cell for the firsttime, and then updated regularly according to the technique described inthe following paragraphs. This measurement will have to be performedagain if the active cell changes, or if a new GSM cell appears in theradio environment of the mobile. Also this measurement has to be carriedout for each GSM cell adjacent to the active UTRAN cell. The estimatedduration T_(offset) will be used for updating the time base of thepassive radio access module with respect to the active radio access celland this is done for each neighboring GSM cell.

The time base is defined here as a set of counters of time measurementunits used in each radio access technique. For example, in GSM, it isnecessary to keep updated the counts of symbols, slots, frames, andmultiframes. These counters constitute the GSM timing unit (GTU) inFIG. 1. Similarly, in UMTS, it is necessary to keep the counts of timemeasurement units of the information transmitted in the current cell.These measurement units are chips, slots, and frames.

The following expression allows for updating the GSM time base from theUMTS time base from the measurement of T_(offset):${GSM\_ Time} = \begin{bmatrix}{\left( {{UTRA\_ fn} \times 10} \right) + \left( {{UTRA\_ sn} \times \frac{10}{15}} \right) +} \\{\left( {{UTRA\_ cn} \times \frac{10}{38400}} \right) - T_{offset}}\end{bmatrix}$where UTRA_fn represents the value of the frame counter, UTRA_snrepresents the value of the slot counter, and UTRA_cn represents thevalue of the chip counter in UMTS.

The value of GSM_Time is then used to estimate the corresponding valuesof a superframe, a frame, a slot and a quarter of GSM bit according tothe following expressions:${GSM\_ sfn} = {{INT}\left\lbrack \frac{GSM\_ Time}{\left( {3060/13} \right)} \right\rbrack}$${GSM\_ fn} = {{{INT}\left\lbrack \frac{GSM\_ Time}{\left( {60/13} \right)} \right\rbrack}{{MOD}(51)}}$${GSM\_ sn} = {{{INT}\left\lbrack \frac{GSM\_ Time}{\left( {15/26} \right)} \right\rbrack}{{MOD}(8)}}$${GSM\_ qb} = {{{INT}\left\lbrack \frac{GSM\_ Time}{\left( {15/16250} \right)} \right\rbrack}{{MOD}(625)}}$

Similarly and given that a GSM frame amounts to 60/13 ms, and that a GSMslot amounts to 15/26 ms and that a quarter of a GSM bit is 15/16250 ms,UMTS base time may be obtained from the GSM base time according to thefollowing expression:${UTRA\_ Time} = \left\lbrack {\left( {{GSM\_ fn} \times \frac{60}{13}} \right) + \left( {{GSM\_ sn} \times \frac{15}{26}} \right) + \left( {{GSM\_ qb} \times \frac{16}{16250}} \right) + \left( {N \times 51 \times \frac{60}{13}} \right) + T_{offset}} \right\rbrack$where GSM_fn, GSM_sn and GSM_qb represent the number of frames, slots,and quarter GSM bits respectively. Thus, we have:${UTRA\_ fn} = {{{INT}\left\lbrack \frac{UTRA\_ time}{10} \right\rbrack}{{MOD}(4096)}}$${UTRA\_ sn} = {{{INT}\left\lbrack \frac{UTRA\_ Time}{\left( {10/15} \right)} \right\rbrack}{{MOD}(15)}}$${UTRA\_ chip} = {{{INT}\left\lbrack \frac{UTRA\_ Time}{\left( {10/38400} \right)} \right\rbrack}{{MOD}(2560)}}$

In operation, the UMTS radio part performs the measurement T_(offset) asspecified by the recommendation 3GPP TS 25.215 with the aid of thedevice represented in FIG. 2. The databases containing time measurementunits 14 and 16 described above are initialized with the measurementT_(offset.)

An exemplary application of the invention is as follows: When the mobileterminal is synchronized with a UMTS cell and the mobile terminal iscommunicating with the network in a dedicated channel, spots areallocated by the network to the mobile where measurements on GSM cellsare to be performed. Then it is necessary for the GSM RAT (radio accesstechnology) to know exactly the start and the end of these spots. Forthis, the UMTS time is converted into GSM time according to theexpressions described above in order to achieve the synchronization andthen to synchronize the two GSM and UMTS radio cards so as to know theexact time to awaken the GSM card while allowing the card to remainsynchronized with respective GSM cells.

Note that, by virtue of the invention, it is not necessary to performthe measurement T_(offset) systematically. In fact, once the valueT_(offset) is measured for the first time as specified by therecommendation 3GPP TS 25.215, the fact that the active radio accessmodule always maintains time synchronization with the network of theactive cell is advantageously used. Thus thereafter it is only necessaryto measure the time offset T_(offset) internally between the activeradio access module and the passive radio access module withoutintervention of signals received from the GSM and UTRAN network. Thedevice in FIG. 2 is used again for this purpose.

Thus, the value T_(offset) is estimated by means of the counter 22 thatis supplied with UMTS and GSM frame timing signals. The start of thecounting is activated by the UMTS frame timing signal and the end of thecounting is controlled by the GSM frame timing signal at the end of aperiod defined by the manufacturer of the terminals (a fewmilliseconds). The precision of the measurement is determined by the19.2 MHz clock. Once the counter is stopped, the difference of timeT_(offset) between the two RAT will be obtained in number of cycles of19.2 MHz clock. The counter is activated by an external signal that maybe generated by the microprocessor of the system.

With reference to FIG. 4, at step 30, the first unit 14 sends to thecentral interface 20 a request for activating the passive radio accessmodule 6. The central interface 20 transmits this request to the passiveto the passive module 6 at step 32. Upon receiving this request, thepassive radio access module 6 sends to the central interface 20 asynchronization demand at step 34. The central interface 20 triggers thecalculation of the time offset T_(diff) by the calculation unit 18 atstep 36. The result of this calculation is then transmitted at step 38to the central interface 20, which transmits it to the first unit 14.The unit 14 generates the synchronization signal GSM_Time for estimatingthe corresponding values of a superframe, a frame, a slot and a quarterGSM bit. The synchronization signal GSM_Time is then transmitted to thecentral interface 20 at step 40, which transmits it at step 42 to thesecond unit 16 designed to maintain the clock signal of the GSM module.

Once the two radio access modules are synchronized by means of thesignal GSM_Time, the intersystem measurement may be carried out at step50.

At the time a cell change, the time T_(offset) is transmitted at step 52to the central interface 20, which transmits it at step 54 to thecalculation unit 18. Upon receiving the value of T_(offset), thecalculation unit 18 performs updating of the T_(offset) and sends backat step 56 the result of the updating to the central interface 20, whichtransmits it at step 58 to the first unit 14. The first unit 14recalculates the synchronization signal GSM_Time and transmits it atstep 60 to the central interface 20, which transmits it at step 62 tothe second unit 16.

1. A method for time-synchronization of at least two radio accessmodules of a multimode communication terminal capable to functionaccording to at least two distinct radio access technique in a cellulartelecommunication network in which one of said radio access modules isactive in a current cell and the other radio access module is in apassive state in said current cell, said method comprising steps of: (a)measuring for each of cells adjacent to the current cell a time offsetT_(offset) between start of a specific frame of the first radiotechnique and start of a specific frame of the second radio accesstechnique; and (b) using the time offset T_(offset) measured in step (a)for synchronizing the passive radio access module with the active radioaccess module.
 2. The method according to claim 1, further comprising astep of activating the passive radio module from the active radio accessmodule.
 3. The method according to claim 1, comprising a step ofupdating a value of the offset T_(offset) on each change of the currentcell and for each neighboring cell associated with the radio accesstechnique of the passive module.
 4. The method according to claim 1,wherein the mobile terminal is a UMTS/GSM dual-mode terminal and whereinthe predefined duration T_(offset) is a time difference observed on GSMdefined in standard 3GPP TS 25.215.
 5. The method according to claim 2,wherein the activation of the passive access module is made immediatelybefore the measurements on cells adjacent to the current cell.
 6. Adevice for time-synchronizing at least two radio access modules of amultimode communication terminal capable to function according to atleast two distinct radio access techniques in a cellulartelecommunication network in which one of said radio access modules isactive in a current cell and the other radio access module is in apassive state in said current cell, said device comprising: means formeasuring a time offset T_(offset) between start of a specific frame ofthe first radio technique and start of a specific frame of the secondradio access technique; and means for synchronizing the passive radioaccess module with the active radio access module using the tine offsetT_(offset.)
 7. A multimode mobile terminal comprising: a radio accessmodule dedicated to each operating mode; a clock generator associatedwith each radio access module; and a unit for calculating a time offsetT_(offset) between start of a specific frame of a first operating modeand start of a specific frame of a second operating mode in a cellulartelecommunication network, wherein said mobile terminal comprises acentral interface capable to generate a clock signal of a passive radioaccess module shifted with respect to a clock signal of an active radioaccess module concerning said duration T_(offset.)
 8. The mobileterminal according to claim 7, wherein said central interface comprisesa module for generating an order for activating the passive radio accessmodule.
 9. The mobile terminal according to claim 7, wherein said mobileterminal supports a UMTS network and a GSM network.
 10. The methodaccording to claim 2, comprising a step of updating a value of theoffset T_(offset) on each change of the current cell and for eachneighboring cell associated with the radio access technique of thepassive module.
 11. The method according to claim 2, wherein the mobileterminal is a UMTS/GSM dual-mode terminal and wherein the predefinedduration T_(offset) is a time difference observed on GSM defined instandard 3GPP TS 25.215.
 12. The method according to claim 3, whereinthe mobile terminal is a UMTS/GSM dual-mode terminal and wherein thepredefined duration T_(offset) is a time difference observed on GSMdefined in standard 3GPP TS 25.215.
 13. The method according to claim10, wherein the mobile terminal is a UMTS/GSM dual-mode terminal andwherein the predefined duration T_(offset) is a time difference observedon GSM defined in standard 3GPP TS 25.215.
 14. The mobile terminalaccording to claim 8, wherein said mobile terminal supports a UMTSnetwork and a GSM network.